This invention pertains generally to pulse Doppler radar tracking systems, and particularly to an improved range gating control circuitry for use in such type of system to position a range gate around echo signals from a moving target.
It is a fundamental precept that the time for a radar pulse to travel to and from a target is a function of the range between a radar and a target. If a target is approaching or moving away from a radar, i.e., if a so-called Doppler velocity is experienced, then the range between the target and the radar changes and a concomitant change in the time of arrival of successive echo signals from the target is experienced. The magnitude of such change in the time of arrival is of particular importance for a pulse radar installed on a guided missile incorporating a range gating technique to reduce the effects of interfering signals, such as returns from other targets in a formation or from clutter. In such an application, for example, if a typical radar dwell is 10 milliseconds and a Doppler velocity of 6,000 feet per second exists, the time of arrival of echo signals will change by 120 nanoseconds from beginning to end of such a dwell. Thus, with a "100 nanosecond" range gate, properly positioned at the beginning of such a radar dwell, the change in the time of arrival of echo signals will exceed the width of the range gate, meaning that echo signals will be lost. A technique, referred to in the art as "velocity-aiding", then must be utilized to change the position of a range gate during a radar dwell in accordance with the Doppler velocity between a target and the guided missile to avoid loss of echo signals.
Known ways to achieve velocity-aiding are subject to the difficulty that undesirable spreading of echo signals from clutter is experienced with an accompanying degradation of resolution. Notwithstanding the spreading of the spectrum of clutter, velocity-aiding is useful, even in the terminal phase of an intercept in which there may be several targets flying in formation. In such a situation, the Doppler frequency of each target is almost identical so that, if in a tight formation, tracking on the centroid of the formation may occur. However, because the ratio of the levels of echo signals from targets and clutter ratio is favorable during the terminal phase of an intercept, velocity-aided range gates may be used to track a particular target in the formation.
One known velocity-aided range gate generator is described in U.S. Pat. No. 4,156,875 issued May 29, 1979 to inventors Keane et al. and assigned to the same assignee as the present invention. Briefly, such a range gate generator is operative to change, by either one of two fixed increments, the interval between the time of each transmitted pulse and the time at which each associated range gate is generated. The increments chosen by patentees is .+-.20 nanoseconds, depending upon the sign of the Doppler velocity. Thus, when the Doppler velocity is positive, indicating that the range between missile and target is decreasing with time, the increment actually chosen is -20 nanoseconds; on the other hand, when the Doppler velocity is negative, the increment actually chosen is +20 nanoseconds. In almost all tactical situations the increment actually chosen is adequate, meaning that coincidence between the echo signal from a target being tracked and each successively generated range gate is maintained. There are, however, tactical situations in which the use of either one of a fixed increment in the order of .+-.20 nanoseconds may not be desirable. For example, if a desired target is an aircraft in a formation of aircraft and tracking is being carried out using a so-called "split-gate", echo signals from aircraft other than the desired target may come into the range gate, especially during the terminal phase of an intercept. At best, then, tracking on the centroid of the targets whose echo signals are in the range gate will take place; at worst, tracking on an unwanted target will occur.